The electronic device industry uses many different types of memory in computers and other electronic systems, such as automobiles and traffic control systems. Different types of memory have different access speeds and different cost per stored bit. For example, items of memory that require rapid recovery may be stored in fast static random access memory (RAM). Information that is likely to be retrieved a very short time after storage may be stored in less expensive dynamic random access memory (DRAM). Large blocks of information may be stored in low cost, but slow access media such as magnetic disk. Each type of memory has benefits and drawbacks, for example DRAMs lose the stored information if the power is shut off. While the magnetic memory can retain the stored information when the power is off (known as non-volatile), the time to retrieve the information is hundreds of times slower than semiconductor memory such as RAM. One type of non-volatile semiconductor memory device is electrically programmable read-only memory (EPROM). There are also electrically-erasable programmable read-only memory (EEPROM) devices. One type of EEPROM is erasable in blocks of memory at one time, and is known as flash memory. Flash memory is non-volatile like magnetic memory, is much faster than magnetic memory like RAM, and is becoming widely used for storing large amounts of data in computers. However, writing information to a conventional flash memory takes a higher write voltage than it does to write information to conventional RAM, and the erase operation in flash requires a relatively long time period.
Conventional EEPROM devices, such as flash memory, may operate by either storing electrons on an electrically isolated transistor gate, known as a floating gate, or not storing electrons on the floating gate. Typically the write (or program) operation and the erase operation are performed by another transistor gate, known as the control gate, which is located above the floating gate. A large positive voltage on the control gate will draw electrons from the substrate through the gate oxide and trap them on the floating gate. The erase operation uses a large negative voltage to drive any stored electrons on the floating gate off of the gate and back into the substrate, thus returning the floating gate to a zero state. This operation may occur through various mechanisms, such as Fowler-Nordheim (FN) tunneling. The rate at which the electrons can be transported through the insulating gate oxide to and from the floating gate is an exponential factor of both the thickness of the insulator and of the electrical height of the insulation barrier between the substrate and the floating gate. Grown gate oxides have great height, and slow tunneling.
Electronic devices have a market driven need to reduce the size and power consumption of the devices, such as by replacing unreliable mechanical memory like magnetic disks, with transistor memory like EEPROM and flash. These increasingly small and reliable memories will likely be used in products such as personal computers (PCs), personal digital assistants (PDAs), mobile telephones, laptop PCs, and even in replacing the slow hard disk drives in full sized computer systems. This is because a solid state device, such as flash memory, is faster, more reliable and has lower power consumption than a complex and delicate mechanical system such as a high speed spinning magnetic disk. What is needed is an improvement in the erase time for EEPROM devices. With improved erase times, the high density of flash memory, and a speed of operation comparable to DRAMs, flash memory might replace both magnetic memory and DRAMs in certain future computer devices and applications.